Vol 21, No 5 (2018)

Numerous experimental studies on shock wave loading of metals have shown by electron microscopy that the crystal structure of the material can undergo transformation in a certain impactor velocity range. At the macroscale, these changes are observed as energy losses associated with the formation of a new structure. The losses are manifested on the time-velocity profile of the rear target surface which contains key information about the material properties. In this paper, a two-component model of a material with a nonlinear internal interaction force is proposed for the description of structural transformations, taking into account the periodic structure of the material. Dynamic equations are written with respect to the motion of the center of mass of the components acting as a measured macroparameter, as well as with respect to their relative displacement serving as the internal degree of freedom responsible for structural transformations. The proposed model is applied to solve a quasi-static problem of the kinematic extension of a two-component rod in order to determine the parameters of a nonmonotonic stressstrain curve, which is often used in describing materials subjected to phase transformations. By solving a dynamic problem of nonstationary impact on the material by a short rectangular pulse, the effect of nonstationary wave damping is demonstrated which is associated with the wave energy dissipation in structural changes of the material. An analytical expression is obtained on the basis of a continuous-discrete analogy for estimating the duration of structural transformations and the parameter characterizing the internal interaction force between the components. The conclusions are confirmed by a numerical solution of a nonlinear Cauchy problem within the finite difference framework.

The paper presents experimental data which substantiate the fundamental role of mesoscopic states in plastic deformation as applied to nanostructured metal materials. Such mesoscopic states in a deformed nanostructured material arise at the interstices of lattice curvature zones due to the loss of its translation invariance, abnormally high concentration of nonequilibrium vacancies, and permanent changes in its electronic subsystem. The main mode of plastic deformation in nanostructured materials is noncrystallographic motion of point defects via plastic distortion. There can be also dynamic rotations, structural phase transitions, and formation of nonequilibrium phases absent in phase diagrams. If nonequilibrium vacancies coalesce, nanostructured materials display low plasticity; otherwise, they become superplastic.

Here, in the context of space, time, and energy, we analyze the nanoscale mesosubstructure of ultrafinegrained nickel and copper after equal channel angular pressing and subsequent rolling and its changes after lowtemperature annealing. The analysis, including scanning tunnel microscopy and positron lifetime spectroscopy, shows that the basis for plastic deformation in such materials is provided by their lattice curvature and associated nanoscale mesoscopic strain-induced defects. Under equal channel angular pressing and rolling, for example, these structural elements increase the role of nonequilibrium point defects, plastic distortion, and low-angle subboundaries. We also analyze the energy of internal interfaces (grain boundaries) estimated from dihedral angles of etch grooves of different scales and their relative energy from cumulative energy distribution functions. In ultrafinegrained nickel, the integral energy distribution function is Gaussian both after equal channel angular and rolling and after further low-temperature annealing, and this is because of the presence of low-angle subboundaries. In ultrafine-grained copper, the integral energy distribution function is Gaussian after equal channel angular pressing and rolling, and after low-temperature annealing it assumes a power form because of the absence of lattice curvature and low-angle subboundaries. Both metals reveal vacancy clusters due to their lattice curvature and to dissolved low-angle subboundaries. In ultrafine-grained copper at T> 180°C, dynamic recrystallization occurs as nonequilibrium low-angle subboundaries inside nanograins are dissolved. It is the lattice curvature that controls the formation and evolution of mesoscopic substructures on different scales under low-temperature annealing.

A multilevel hierarchical mesosubstructure is experimentally shown to form in metastable austenitic steel subjected to multipass cross rolling in the temperature range 950-75№C. A multilayer mesosubstructure with highly refined grains, changed grain geometry and microhardness builds up between the macro- and microlevels. Its outer layer has a finely dispersed structure and the highest microhardness H_μ = 3400 MPa. The two underlying layers are composed of globular grains 0.9 μm in diameter and exhibit the respective close microhardnesses 3100 and 3000 MPa. The near-axial layer has elongated grains of a fiber-band structure with nanosized width. Between the micro- and nanolevels, a nanosized hierarchical mesosubstructure is formed in all band structures, with the mesosubstructure of nanosized carbides and cardonitrides precipitated at low-angle boundaries. Despite a certain decrease in the steel plasticity in uniaxial tension, such a multilevel mesosubstructure causes a multiple increase in its fatigue life. The formation of hierarchical mesosubstructures is associated with the mechanism of plastic distortion under the conditions of the crystal lattice curvature, when bifurcational structural states arise in its interstices. Multilevel hierarchical mesosubstructures have a strong positive effect on the mechanical behavior of solids, by increasing their strength, wear resistance, and fatigue life.

Ultrasonic impact treatment (UIT) of alloy Ti-6Al-4V (VT6) causes a high lattice curvature, nanostructuring of thin surface layers, and the formation of complex band structures of T Al pre-precipitates in the a phase of the underlying sublayer, as well as the formation of the martensitic a ' phase. In so doing, the fatigue life of the alloy increases only by a factor of 1.3 due to the negative influence of complex band structures. Positron annihilation spectroscopy revealed a nonequilibrium vacancy concentration in the treated surface layer equal to 10^-5, which is by five orders of magnitude greater than the equilibrium vacancy concentration. This makes possible reversible structural transformations through plastic distortion under cyclic loading of VT6 and underlies the increase in fatigue life. There is a convergence of the electron energy distribution curves for VT6 + UIT and Al obtained from the Doppler broadening spectra of annihilation radiation. This result suggests the formation of Ti-Ti-Al clusters and Ti₃Al pre-precipitates in etch-resistant banded structures. Hydrogen charging of the ultrasonically treated VT6 surface layers leads to a 4-fold decrease in the fatigue life of the material. This effect is due to the formation of α"-phase martensite laths in the a phase which rearranges the hcp lattice into an orthorhombic structure under the functional influence of hydrogen, with the segregation of vanadium atoms in the α"-phase bands. The segregation causes a convergence of the electron energy distribution curves of VT6 + UIT + HN and V, as evidenced by the Doppler broadening spectra of annihilation radiation. Bundles of α"-phase bands reinforce the nanostructured surface layer, which drastically reduces the fatigue life of the alloy. Its microhardness in the zone of fatigue fracture greatly increases. The multiscale structural analysis of fatigue fracture is carried out on the basis of the mesomechanical space-time-energy approach.